1. Field of the Invention
This invention generally relates to electronic signal modulation and, more particularly, to spread spectrum symbol timing in Harmonic Modulation (HM) power line communications (PLC).
2. Background Description
Electric power companies typically have a service person visit customers and read a power meter to determine the amount of power consumed for billing purposes. This can be cumbersome and time consuming when there are many customers, especially if they are dispersed over a large geographic area.
It would be beneficial for power companies to read these electric power meters remotely. The power wires connected to each customer's power meter constitute a possible path for communicating with the power meters; however, existing modulation techniques may not be easily implemented or reliably perform in the high-noise environment of a power line. Moreover, transformers employed in the power line network are inherently inductively coupled and consequently introduce non-linear phase shifts in signals passing through the transformer. Therefore, complex communications systems are required to perform reliably in the high-noise, phase distorted power line channels. For example, some of these systems monitor several frequency channels, select an appropriate channel, and then indicate the appropriate frequency to other communicating units. Other systems, such as that set forth in U.S. Pat. No. 5,185,591 to Shuey, issued Feb. 9, 1993, employ a plurality of signals which are not harmonically related, so as to require elaborate filtering and signal extraction.
Currently, there is need for a relatively simple communication system which can utilize existing power lines to remotely read multiple power meters simultaneously.